The present invention relates in general to the field of bathymetry, that is the measurement of ocean depths, and in particular to a new and useful method of steering the return beam of a receiver which receives reflective energy from a target that has been illuminated by a transmitted beam of energy. The transmitted and reflected energy is generally acoustical energy and the return beam is formed by combining and processing signals from a plurality of transducing elements lying in an array. The array of transducing elements is divided into at least two groups, with the signals of each group being processed to form distinct beams for each group. Knowledge of a prior steering direction for the target is used to initially steer the beams of each group. A small offset from the known steering angle is then applied to each beam. The invention takes advantage of an observation that the phases of the beams, within certain limits, change in a linear fashion with steering angle. An error or deviation of steering angle between the actual target and the previous steering angle can thus be reduced to zero by finding the intersection of lines representing phase versus steering angle.
A method and apparatus for mapping the contours of the ocean bottom is known from U.S. Pat. No. 3,144,631 to Lustig et al. A fan-shaped beam of acoustical energy is emitted downwardly toward the ocean bottom. The fan is narrow in the travel direction of a ship carrying the mapping equipment, and is broad in a transverse direction perpendicular to the travel direction. In this way, as the ship moves over the ocean bottom, a swath of ocean bottom is ensonified. The echoed or back scattered return of acoustical energy is sensed by a receiver which comprises a plurality of individual transducing elements lying in an array which extends in the transverse direction. Signals from each transducing element are combined and processed to form return beams. Each return beam is elongated in the travel direction and narrow in the transverse direction. A plurality of these return beams are formed across the width of the illuminated strip of ocean bottom. Each return beam is steered to a different and constant steering angle so that the entire illuminated area is covered by return beams. This beamforming technique uses algorithms based on temporal detection of the output of the preformed return beams. The algorithm has been realized as analogue electronic circuits, in a computer program and as combinations of both.
The outputs of this beamforming process are passed to detection circuits which process the outputs in time to obtain best estimates of when the bottom was ensonified in each of the preformed beams. These estimates are then corrected for the refraction of water, ships motion and offset to obtain final, precise bathymetric measurements. These can be displayed to show the actual contour of the ocean bottom as it passes under a moving ship carrying the bathymetry equipment.
A contour mapping method using similar techniques is disclosed by U.S. Pat. No. 3,191,170 to Lustig et al. In this reference, the transmitting and receiving equipment is mounted on an aeroplane and is used to measure the contour of a swath of ground over which the aeroplane moves.
U.S. Pat. No. 3,641,484 to White et al discloses a technique similar to the Lustig et al patents, which however, better compensates for rolling and pitching of a vehicle carrying the mapping equipment.
U.S. Pat. No. 4,611,313 to Ziese discloses a further refinement of the swath bathymetric system which compensates for differences in refraction of sound waves through the water due to layers of water having different temperatures which lie between a ship carrying the surveying equipment and the ocean bottom.
While the foregoing patents and the present invention disclose the details of acoustic surveying and sounding equipment, it is understood that other forms of energy (e.g. radio waves) can be used while still applying the principles of the references and the present invention.
The prior art as exemplified by the foregoing references utilize algorithms which lead to inefficiencies in that they utilize preformed beams at preselected look angles. Since there is never any doubt that the ocean bottom or earth's surface is actually present, the modification of detection algorithms where there is no detection problem is inefficient. In particular, the bathymetric problem is rather to extract certain features of the return signal which characterize the bottom. An algorithm which assumes the presence of the bottom and concentrates on extracting bathymetric information, thus would prove superior to known detection methods.